The present invention relates generally to tunable electronic devices and components and, more particularly, to crystal oscillators incorporating tunable ferroelectric components.
Radio frequency bandwidth is a scarce resource that is highly valued and is becoming increasingly congested. Ever-increasing numbers of users are attempting to co-exist and to pass ever-increasing amounts of information through the finite amount of bandwidth that is available. The radio spectrum is divided into frequency bands that are allocated for specific uses. In the United States, for example, all FM radio stations transmit in the 88-108 MHz band and all AM radio stations transmit in the 535 kHz-1.7 MHz band. The frequency band around 900 MHz is reserved for wireless phone transmissions. A frequency band centered around 2.45 GHz has been set aside for the new Bluetooth technology. Hundreds of other wireless technologies have their own band of the radio spectrum set aside, from baby monitors to deep space communications.
Communications within a given frequency band occur on even more narrowly and precisely defined channels within that band. Hence, in virtually any wireless communication system or device, frequency agility is required and accurate frequency generation is of critical importance. Voltage controlled oscillators (VCOs) can generate a large range of frequencies, but are problematic in that they are unstable and tend to drift. Crystal oscillators, conversely, provide excellent frequency stability but can be used only over the narrow frequency range of the particular quartz crystal contained in the oscillator. A phase-locked loop (PLL) is a commonly used frequency synthesis technique that takes advantage of both the flexibility of a voltage controlled oscillator and the stability of a crystal oscillator.
FIG. 1 depicts a typical frequency synthesis circuit including a PLL 100 and reference crystal oscillator 200. PLL 100 operates in a well-known fashion. Briefly, VCO 108 generates a range of output frequencies via variation of a DC control voltage input to the VCO. The output of VCO 108 is fed back to phase comparator 102, which also receives an input from crystal oscillator 200. The VCO and crystal oscillator inputs to comparator 102 are combined, and any difference in phase/frequency results in a DC voltage output. The output of comparator 102 is coupled back to the input of VCO 108 via charge pump 104 and loop filter 106. The greater the frequency/phase difference that exists between the two signals, the larger the output voltage from comparator 102, and the larger the adjustment that is made to the VCO output frequency. Hence, the output of VCO 108 is driven to, and eventually locks onto, the frequency of crystal oscillator 200. Divider 110 is positioned in the feedback path between VCO 108 and phase comparator 102. It is typically a programmable frequency divider that divides the VCO frequency down to the reference frequency produced by crystal oscillator 200. Hence, a large range of output frequencies can be generated and frequency lock still maintained by manipulation of the divide-by number.
Crystal oscillator 200 uses a quartz crystal to provide a fixed and stable reference frequency. Since temperature affects the rate at which a crystal vibrates, crystal oscillators often include a temperature compensation network that senses the ambient temperature and xe2x80x9cpullsxe2x80x9d the crystal frequency to prevent frequency drift over a temperature range. A temperature compensated crystal oscillator is referred to as a TCXO. Crystal oscillators, like voltage controlled oscillators, can also be made tunable over the frequency range (albeit much smaller) of the crystal by application of a control voltage. A voltage controlled crystal oscillator is referred to as a VCXO. A crystal oscillator that combines the attributes of voltage control and temperature compensation is referred to as a VC-TCXO.
In recent years, VC-TCXO designs have been required to comply with significantly more demanding specifications. VC-TCXOs are typically required to remain frequency stable to within xc2x12.5 parts per million (ppm) over a temperature range of xe2x88x9230xc2x0 C. to +85xc2x0 C. Moreover, their initial accuracy or tolerance at ambient temperature must be within xc2x11.5 ppm.
Modern communication devices also impose very high spectral purity and phase noise demands on the VC-TCXO. Phase noise affects the receiver""s ability to reject unwanted signals on nearby channels. It is the ratio of the output power divided by the noise power at a specified offset and is expressed in dBc/Hz. The reference frequency produced by a VC-TCXO may be used to phase lock an output frequency that is one-hundred times or more greater. Since phase noise performance is degraded by a factor of 20 log N, where N is the frequency multiplier or divide-by number, close in phase noise performance (i.e. at offsets of less than 100 Hz) exceeding xe2x88x9290 dBc is required.
Close in phase noise performance is especially important in FM systems and in position location systems. In FM systems, close in phase noise directly degrades the audio quality and manifests itself as an audio hiss. In position location systems, such as those used to receive GPS signals, close in phase noise may manifest itself as positional inaccuracy or receiver sensitivity impairment. Receiver sensitivity impairment is due to the short term frequency or phase jitter of the reference oscillator caused by the phase noise, and the subsequent inability of the receiver to accurately correlate for extended periods of time.
To meet these stringent accuracy and close in phase noise requirements, the VC-TCXO topology and components must be chosen with caution. The use of a dedicated integrated circuit (IC), a frequency control element and factory calibration are required. Each of these requirements adds considerable time and cost to production. Currently, only a handful of manufacturers world wide can economically produce VC-TCXOs meeting these requirements that are suitable for use in high volume consumer communication devices.
The Q of the oscillator quartz crystal alone can be as high as 50,000, meaning that additional losses of any significance at all in the resonator network will dramatically reduce the overall Q and seriously degrade close in phase noise performance. One element that is particularly difficult to implement and control without degrading close in phase noise performance is the voltage controlled, frequency-tuning element associated with the crystal. In a VC-TCXO, this element usually takes the form of a voltage dependent capacitor, commonly referred to as a variable capacitance diode, varicap diode or varactor.
The operation of a varactor is well understood. When a reverse voltage is applied to a varactor, the insulation layer between the p-doped and n-doped semiconductor regions thickens. A depletion region that is essentially devoid of carriers forms and behaves as the dielectric of the capacitor. The depletion region increases as the reverse voltage across it increases, and since capacitance varies inversely as dielectric thickness, the junction capacitance decreases as the reverse voltage increases. Capacitance variations effected by control voltage variations effect corresponding variations in the resonant frequency of the oscillator.
Unfortunately, a typical varactor has a relatively low Q due to its intrinsic series resistance, which may be as much as several ohms. The Q of a varactor can be expressed as Xc/Rs, where Xc is the reactance of the varactor (1/[2xc2x7xcfx80xc2x7fxc2x7c]) and Rs is the effective series resistance of the varactor. A capacitance of 5 pF at a frequency of 1.5 GHz results in a reactance Xc of 21.22 xcexa9. If the effective series resistance Rs of the varactor is 0.5 xcexa9, the resultant Q is 42.44. When compared to the extremely high Q of the crystal itself, it can be seen how much this will degrade the overall Q of the oscillator. Clearly, the effective series resistance of the varactor poses a problem and a tuning element having less series resistance will have a highly direct and positive impact on the overall Q.
Another problem with use of varactors is their non-linear transfer function (applied voltage versus capacitance). FIG. 2 is a chart plotting the capacitance of a typical varactor versus a common tuning range for this component in a mobile phone (0.3V to 2.7V). As can be seen, it is not a linear relation. Below 0.5V, unit voltage changes lead to much greater unit capacitance changes. Consequently, the MHz/volt frequency shift of the oscillator is not constant across the tuning range.
Another problem associated with the varactor is that, since it is a reverse-biased diode junction, it is important that the applied AC signal does not overcome the bias voltage and result in heavy forward conduction of the diode. If this occurs the Q of the resonator will be dramatically lowered and various oscillator parameters such as phase noise and general spectral purity will be seriously impacted. In extreme cases, the oscillator may fail to maintain a continuous oscillation and degenerate into parasitic uncontrolled burst oscillations.
In view of the above, there is a need for a voltage controlled temperature compensated crystal oscillator employing frequency tuning elements without the attendant drawbacks of degraded phase noise performance and non-linear transfer function.
The present invention provides a voltage controlled temperature compensated crystal oscillator that incorporates a tunable ferroelectric capacitor to provide better phase noise performance and a more linear voltage/capacitance transfer function.
Accordingly, one embodiment of the invention provides a voltage controlled crystal oscillator. It comprises a crystal for generating a reference frequency, a ferroelectric capacitor having a variable capacitance, and a control line coupled to the ferroelectric capacitor for applying a control voltage to the capacitor. The control voltage varies the capacitance which varies the reference frequency generated by the crystal.
Another embodiment of the invention provides a temperature compensated crystal oscillator. It comprises a crystal for generating a reference frequency, a ferroelectric capacitor having a variable capacitance, a temperature sensor and temperature compensation control circuitry coupled to the temperature sensor and to the ferroelectric capacitor. The control circuitry applies an appropriate correction voltage to the ferroelectric capacitor in response to changes in temperature sensed by the temperature sensor. The correction voltage varies the capacitance of the ferroelectric capacitor which, in turn, varies the reference frequency generated by the crystal.
Another embodiment of the invention provides a crystal oscillator. It comprises a crystal for generating a reference frequency and a ferroelectric capacitor having a capacitance that is adjusted and then fixed during manufacture to set the initial tolerance of the crystal oscillator at ambient temperature.
Another embodiment of the invention provides a voltage controlled temperature compensated crystal oscillator. A fine tuning ferroelectric has a variable capacitance and is coupled to an automatic frequency control line that applies a control voltage to vary the capacitance of the fine tuning capacitor which, in turn, fine tunes the reference frequency generated by the crystal. A temperature compensating ferroelectric capacitor has a variable capacitance and is coupled to a temperature sensor and temperature compensation control circuitry. The control circuitry applies an appropriate correction voltage to the temperature compensating capacitor in response to changes in temperature sensed by the temperature sensor to vary the capacitance of the temperature compensating capacitor which, in turn, varies the reference frequency generated by the crystal in response to ambient temperature changes. A third ferroelectric capacitor has a capacitance that is adjusted and then fixed during manufacture to set the initial tolerance of the crystal oscillator at ambient temperature.
Other features, objects and implementations of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. All such additional features, objects and implementations are intended to be included within this description, to be within the scope of the invention and to be protected by the accompanying claims.